1. Field of the Invention
This invention relates to a method and an apparatus for etching uniformly the surface of a semiconductor single crystal wafer (hereinafter referred to simply as "wafer") obtained by slicing a semiconductor single crystal rod such as silicon and then lapping the opposite surfaces of each wafer resulting from the slicing to impart to the wafer an etched surface of high flatness.
2. Description of the Prior Art
The chemical etching of wafer is utilized for the purpose of (1) removing from the surface of a wafer a deformed layer produced by such machining works as slicing and lapping, (2) allowing visual observation of crystal defects, and (3) microprocessing a wafer by selective etching, for example. The present invention is aimed to accomplish the purpose of (1) mentioned above.
The conventional method used for etching of wafer consists in etching a given wafer by rotating the wafer in the flow of etchant caused as by bubbling and enabling the etchant in motion to come into contact with the surface of the wafer. In this case, the rotation of the wafer is intended to flatten the wafer surface as much as possible by abating the possibility that the reaction gas arising during the etching treatment will impart streaks or random undulation to the wafer surface.
Incidentally, in the etching treatment, the desirability of decreasing the inconstancy of etching depth at various parts of the wafer or improvement the flatness of wafer surface is never fulfilled unless the relative average flow rate of the etchant and the wafer with which the etchant contacts is uniformized as much as possible at various parts of the wafer.
The conventional method mentioned above, however, it is difficult to control the flow rate of the etchant, so it fails to uniform the etching depth of the wafer surface and to obtain a chemically etched wafer having high flatness.
In the etching treatment, the etchant first contacts on the outer edge part of the wafer and then flows parallelly with the main surface of the wafer to the central part of the wafer. The relative average flow rate of the etchant and the wafer is locally varied in the outer edge part mentioned above. As a result, the etching treatment tends to become incomplete particularly in the edge part. FIG. 9 is intended to depict the state of incomplete etching.
When the etchant flows in the direction of the arrow B while the wafer W is kept rotating,or not rotating as shown in the upper part of FIG. 9, the flow rate varies at the edge part as mentioned above and a locally abnormal shape 101 occurs in the edge part of the wafer W as shown in the lower part of FIG. 9. Since this abnormal shaping also affects the neighboring region of the wafer, the wafer W is not uniformly etched and therefore is prevented from acquiring a flat surface. FIG. 9 illustrates the abnormal shape on a magnified scale. In the diagram, the vertical axis represents the scale of thickness of the wafer W and the horizontal axis the scale of distance in the radial direction.